Currently there is a great interest in manufacturing printed circuits, sensors, electronics and radio frequency identification tags via liquid based “ink”. Generally, the functional ink is made of metallic nanoparticles (NPs) dispersed in organic/inorganic solvents. By using the functional fluid, inkjet printing technology is a promising method to deposit metallic nanoparticles on polymer substrate with designed patterns, however, there are many problems associated with the use of inkjet printing that need to be addressed.
The most widely used inks are made of silver and gold noble NPs due to the low melting temperature and chemical stability at a high temperature while sintered in air as well as high electrical conductivity. Also used NP inks are copper, platinum, and nickel organic/aqueous suspensions.
However, the utilization of such NP inks has disadvantages. Gold NPs are very expensive to utilize in mass production. Also NP based ink requires a high sintering temperature to join particles. Also, nanoparticle based ink is not suitable to high resolution printing due to nozzle clogging during printing.
Constantan (Cu55Ni45) nickel-copper alloy is also known for its excellent thermoelectrical properties. Nickel is much less expensive than gold, however, there are still problems using constantan based ink in printing conductive or electronic items. Constantan may be used for precision resistors, thermocouples and strain gauges with low temperature coefficient of resistivity (TCR). However, this material suffers surface oxidation that has been a serious impediment to the development and application of constantan in inkjet printing technology. Also, high melting temperature of constantan (1221˜1300° C.) makes it hard to sinter especially at low temperature such as below 400° C. The sintering process of constantan usually requires a reducing atmosphere which is flammable gas, for instance hydrogen and methane. Reducing gas with high temperature is hard to achieve and dangerous in industry.
Therefore there still exists a critical need for a safe process to anneal (sinter) the inkjet printed constantan patterns at low temperature without using flammable reducing gas. Also there is a need for a NP ink that avoids nozzle clogging, uses lower sintering temperatures and achieves high resolution printing.